What's the Expected Time for Signs of Feline Hyperthyroidism to Resolve after Treatment?

My 16-year old cat was recently treated with radioiodine (I-131) for his severe hyperthyroidism. How long after radioiodine treatment will the thyroid values normalize and the symptoms of the hyperactive thyroid begin to resolve so he feels better? 

My cat lost about half of his body weight, much of it in muscle mass. Will his wasted muscles ever return to normal? If so, when can I expect to see improvement?

My Response: 

You have asked two good questions, which, on the surface seem simple enough to answer. The answers to "how long for clinical signs to resolve" however, depend on a number of factors. I'll do my best to explain why it's not possible for me to give you the definitive answers you want.

How long does it take for serum thyroid hormone levels to normalize after I-131?
Depending on the dosing protocol used, about 90% of cats will have serum thyroid hormone concentrations (e.g., T4 and T3) within reference range limits by 30 days after I-131 treatment. Most of the remaining cats will show a nice drop in T4 and T3 levels when rechecked in a month, but it will take longer to for their thyroid hormone concentrations to completely normalize.

In general, the full extent of the radioiodine treatment will be evident by 3 months after treatment, although a few cats continue to show even more (minor) improvement when rechecked at 4 to 6 months.

As the thyroid values normalize, the clinical signs we see also gradually resolve. Some signs, such as nervousness or rapid heart rate, generally resolve fairly quickly, whereas other signs, such as marked weight loss and muscle wasting, obviously take much longer.

How fast do we want the serum thyroid values to fall after radioiodine treatment?
My goal in treating hyperthyroid cats with radioiodine is to gradually normalize the high serum thyroid hormone concentrations— not lower the values too quickly. I'd rather that the thyroid values fall slowly over the first month after treatment, allowing the rest of the body to gradually get used to being euthyroid once again. This is especially true in cats with concurrent kidney disease, when a drastic fall in thyroid values can aggravate the serum kidney values and can even lead to severe renal failure.

To achieve this gradual fall in the high serum thyroid hormone levels, I administer the smallest dose necessary to cure the hyperthyroidism. By giving individualized, lower doses of radioiodine, we can also reduce the incidence of post-treatment hypothyroidism (underactive thyroid condition) in these cats.

How often does the radioiodine fail to cure the hyperthyroidism?
About 5% of cats that I treat will remain slightly hyperthyroid at the 3-month follow-up period. Many treatment facilities will claim a higher rate of cure (98-100%) than I do, which is made possible by administering higher doses of radioiodine to their cats. In addition to decreasing the incidence of persistent hyperthyroidism, the use of higher radioiodine doses will also hasten the rate of decline in the serum thyroid hormone concentrations.

So why not use this high-dose I-131 protocol instead of my lower-dose approach? Simple — the downside of administering higher radioiodine doses is that this method will lead to a higher rate of iatrogenic hypothyroidism as both the thyroid tumor, as well as most normal thyroid tissue, are irradiated and destroyed (1,2). As I've previously discussed (see my post, Estimating the Radioiodine Dose to Administer to Cats with Hyperthyroidism), more that 30% of cats will become hypothyroid using the standard high-dose treatment protocol, but this incidence could in fact be much higher, possibly up to 75%.

Diagnosing iatrogenic hypothyroidism
To monitor for iatrogenic hypothyroidism, we routinely run a serum thyroid panel (i.e., total T4, T3, free T4, and TSH) at 1 and 3 months after treatment (2,3). Most facilities recommend monitoring just the total T4 concentration, but this is not adequate for monitoring since many hypothyroid cats will maintain a low-normal total T4 value, despite being hypothyroid.  Based on our studies, it's becoming increasing clear that feline hypothyroidism can only be diagnosed by finding low to low-normal T4 and T3 values in conjunction with high TSH values.

Restoring lost body weight and muscle mass
Once euthyroidism is reestablished, most cats will gain weight within a few weeks (certainly by 2-3 months). If marked muscle wasting has occurred, it may not be possible to completely regain the lost muscle. Remember that it will help to feed a diet that's higher in protein (40-50% of calories), higher in fat (40-50% of calories), and relatively low in carbs (less than 15% of calories) (4). For more information, check out my post on The Best Diet to Feed Hyperthyroid Cats.

References:

1. Peterson ME, Broome MR. Radioiodine for feline hyperthyroidism In: Bonagura JD, Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2014.
2. Peterson ME. Feline focus: Diagnostic testing for feline thyroid disease: hypothyroidism. Compend Contin Educ Vet 2013;35:E4. 
3. Peterson ME. Diagnosis and management of iatrogenic hypothyroidism In: Little SE, ed. August's Consultations in Feline Internal Medicine: Elsevier, 2014;in press.
4. Peterson ME. Nutritional management of endocrine disease in cats. Proceedings of the Royal Canin Feline Medicine Symposium 2013;23-28.

Related Blog Posts:

• Peterson ME. How Does Radioiodine Really Work to Treat Hyperthyroidism? Insights into Veterinary Endocrinology blog.  July 12, 2012.
• Peterson MEL Estimating the Radioiodine Dose to Administer to Cats with Hyperthyroidism. Animal Endocrine Clinic blog. April 30, 2013.
• Peterson ME. Success Rates for Radioiodine Treatment in Hyperthyroid Cats. Animal Endocrine Clinic blog. May 24, 2013.
• Peterson ME. The Best Diet to Feed Hyperthyroid Cats. Insights into Veterinary Endocrinology blog. September 13, 2011